3500 Huong Thi Anh Nguyen1, Pavel Kubánˇ2, Viet Hung Pham1 Peter C Hauser2 Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Hanoi, Vietnam Department of Chemistry, University of Basel, Basel, Switzerland Institute of Analytical Chemistry, Academy of Sciences of the Czech Republic, Brno, Czech Republic Received February 2, 2007 Revised April 4, 2007 Accepted April 4, 2007 Electrophoresis 2007, 28, 3500–3506 Research Article Study of the determination of inorganic arsenic species by CE with capacitively coupled contactless conductivity detection The determination of arsenic(III) and arsenic(V), as inorganic arsenite and arsenate, was investigated by CE with capacitively coupled contactless conductivity detection (CE-C4D) It was found necessary to determine the two inorganic arsenic species separately employing two different electrolyte systems Electrolyte solutions consisting of 50 mM CAPS/2 mM L-arginine (Arg) (pH 9.0) and of 45 mM acetic acid (pH 3.2) were used for arsenic(III) and arsenic(V) determinations, respectively Detection limits of 0.29 and 0.15 mM were achieved for As(III) and As(V), respectively by using large-volume injection to maximize the sensitivity The analysis of contaminated well water samples from Vietnam is demonstrated Keywords: Capacitively coupled contactless conductivity detection / CE / Inorganic arsenic ions / Large-volume injection DOI 10.1002/elps.200700069 Introduction CE is a very useful tool in heavy metal analysis due to the low consumption of reagents and consumables and to the fact that short analysis times, high separation efficiencies, and low operating costs are achieved In particular, for speciation the method is significantly less expensive then the oftenused combination of chromatography with inductively coupled plasma (ICP) spectroscopy Several review articles on the applications of CE in metal speciation have been published in the last few years [1–5] as well as specific reviews dealing with the determination of arsenic [6–8] The two inorganic forms of arsenic, arsenite, and arsenate, have been found to be more toxic than the organic arsenic compounds The widespread occurrence of these species has created a strong demand for their monitoring Toxic levels of arsenic in groundwater samples have been found not only in developing countries of South and Southeast Asia but also in South America, the United States of America, and Europe [9] The maximum tolerated contaminant level of all arsenic species in drinking water had been Correspondence: Professor Peter C Hauser, Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4004 Basel, Switzerland E-mail: Peter.Hauser@unibas.ch Fax: 141-61-267-1013 Abbreviations: Arg, L-arginine; C4D, capacitively coupled contactless conductivity detection © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim set to 50 mg/L by the EPA for the time period between 1975 and 2006, but it was lowered to 10 mg/L (0.13 mM) in January 2006 [8] The first analysis of arsenic species (inorganic As(III) and As(V)) by CE was described by Wildman et al [10] using indirect UV detection Standard solutions containing a set of common inorganic anions and the two arsenic species were used for the optimization of the BGE solution and for the subsequent determination of As(V) and As(III) in a spiked urine sample Note, however, that the sensitivity of the method was relatively poor, reaching LOD in the mg/L range and could, therefore, not be used for the analysis of real samples Several other methods based on indirect UV-detection, which all had low detection sensitivity, were reported in subsequent years [11–14] Alternative detection methods were investigated in order to achieve higher sensitivities, and X-ray [15], indirect LIF [16, 17], and conductivity [18] detection modes were shown to be suitable analytical tools for arsenic speciation The sensitivity was, however, in the same range as for indirect UV detection with the exception of conductivity measurements where LODs as low as 60 mg/L were achieved for As(V) Mass spectrometric detection was also applied for the CE determination of arsenic species [14, 19]; the sensitivity, in comparison with other detection methods, was slightly higher for organic species, however, extremely poor LODs were achieved for inorganic arsenic A slightly better sensitivity was obtained when direct UV-detection was applied Arsenic species absorb strongly in low UV-range and detection at wavelengths between 190 and 210 nm has been used for speciation purposes [20–26] Interfering ions (e.g., www.electrophoresis-journal.com CE and CEC Electrophoresis 2007, 28, 3500–3506 inorganic anions and cations in water samples) not absorb in this region (except for nitrate and nitrite) and matrix effects of samples containing high concentrations of inorganic ions (e.g., Cl–, SO42–, Ca21 and Na1) can thus be minimized The determination of several arsenic species in real samples was demonstrated [22–27], although preconcentration techniques, such as large-volume injection and field-amplified sample stacking, had to be applied in most cases For the determination of arsenic species, CE has also been hyphenated to the ICP, however, the sensitivity of the optical detection method [28, 29] did not allow to reach the mg/L concentration levels which are required for real sample analyses So far, the most sensitive detection for arsenic speciation in CE has been achieved with the ICP-MS (see, e.g., the following reviews [30–32]), which unfortunately is not suitable for wide use due to its high cost In the past decade, yet another detection scheme for CE was presented, namely capacitively coupled contactless conductivity detection (C4D) [33–36] This method is simpler than the common UV-detection, and significantly less complex and expensive than ICP-MS Good sensitivity has been achieved for determination via C4D for most inorganic species Several comprehensive reviews on C4D in CE have been presented in recent years [37–39] and the theoretical principles of C4D have been described in detail, for example, in the following articles [40, 41] In this contribution, new methods were developed for the CE-C4D determination of inorganic arsenic species in water samples from Vietnam Serious pollution of ground and drinking water is found for areas in North Vietnam with arsenic concentrations significantly exceeding the EPA limits of 10 mg/L, the most abundant species being inorganic arsenic As(III) and As(V) Large-volume injection was used to achieve LODs close to the regulatory limits and to determine inorganic arsenic pollutants in real water samples Materials and methods 3501 acquisition system (AD Instruments, Castle Hill, Australia) for recording of the electropherograms pH measurements were carried out with a model 744 pH meter from Metrohm (Herisau, Switzerland) 2.2 Reagents and methods All chemicals were of analytical reagent grade and deionized water was used throughout Stock solutions (10 mM) of As(III) and As(V) were prepared from sodium arsenite (Fluka, Buchs, Switzerland) and disodium hydrogen arsenate (Merck, Darmstadt, Germany), respectively Stock solutions of inorganic anions (10 mM) were prepared from the corresponding sodium or potassium salts (Fluka or Merck) All multi-ion standard and calibration solutions were prepared from these stock solutions MES, TAPS, CHES, MOPS, CAPS, L-arginine (Arg), CTAB, and acetic acid (99.8%) were obtained from Fluka or Merck Fused-silica capillaries of 50 mm id and 375 mm od (purchased from Polymicro Technologies, Phoenix, AZ, USA) were used for the electrophoretic separations The total length of the separation capillaries was 50 cm (Leff = 43 cm) and 75 cm (Leff = 68 cm) and the capillaries were preconditioned with M NaOH for 10 min, deionized water for min, M HCl for 10 min, deionized water for 10 min, and finally with electrolyte solution for 15 Electrolyte solutions were prepared daily from the corresponding pure chemicals 10 mM CTAB was prepared as stock solution CAPS and Arg were weighed into a 25 mL volumetric flask and dissolved in 10 mL of deionized water CTAB (30 mM) was added to the volumetric flask from a corresponding stock solution and the flask was filled to the mark with deionized water The electrolyte solution was equilibrated for 15 Electrolyte solutions containing acetic acid were prepared directly from concentrated acid by dilution with deionized water Electrolyte solutions were degassed in an ultrasonic bath for and filtered through a 0.20 mm nylon syringe filter (BGB Analytik, Böckten, Switzerland) Resolution values, R, were calculated according to the standard formula (see, e.g., [42]) 2.1 Instrumentation Separations were carried out on a purpose-made instrument which is based on a high-voltage power supply with 630 kV interchangeable polarity (CZE 2000R) from Start Spellman (Pulborough, UK) The contactless conductivity detector consists of two tubular electrodes of mm length separated by a gap of mm and a Faradaic shield [35, 36] Cell excitation was carried out using a sine wave with a frequency of 200 kHz generated by an external function generator (GFG8216A from GW Instek, Tucheng City, Taiwan) and was boosted to a peak-to-peak amplitude of 300 Vp–p The resulting current signal was converted to voltage using an OPA655 operational amplifier (Texas Instruments, Dallas, TX, USA), amplified, rectified, and low pass filtered with a circuitry described elsewhere [36] before passing to a MacLab/4e data © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Results and discussion 3.1 Analytical procedure for the determination of As(III) 3.1.1 Electrolyte system The pKa1-value of arsenous acid (H3AsO3), the form in which inorganic As(III) is present in aqueous solutions, is 9.2 Thus, the pH value of an electrolyte solution used for the electrophoretic determination of arsenic(III) should be at least 9.0 or higher in order to render a substantial fraction of the analyte in the charged form Arg was chosen as basic component for the preparation of electrolyte solutions because of its high pKa-value (12.5) and low conductivity in www.electrophoresis-journal.com 3502 H T A Nguyen et al aqueous solutions As acidic counter-ions five different species were selected, namely MES, CHES, MOPS, TAPS, CAPS, and the effect of electrolyte solutions consisting of either of these and Arg in each case on the electrophoretic determination of 100 mM arsenic(III) was investigated All electrolyte solutions had a pH value of pH 9.0 To enable the determination of As(III) as the arsenous anion concurrent with the EOF, CTAB, which was found to be compatible with conductivity detection [43], was added to the electrolyte solutions The resulting electropherograms for the determination of arsenic(III) in these electrolyte solutions are shown in Fig Clearly, the composition of the electrolyte solution has a pronounced effect on the sensitivity The direction of the peak deviation from the baseline is dependent on whether the ion of the same charge contained in the buffer has a lower or higher molar conductivity than the analyte ion, and both orientations are acceptable in conductivity detection Note that fractional charges due to partial dissociation also have a bearing on the effective conductivity of the species The system peak found in all electropherograms was associated with the presence of CTAB and the carbonate peak (mostly due to bicarbonate) is due to absorption of ambient CO2 from air into the solutions The S/N ratios were evaluated for the electrophoretic determination of As(III) in the electrolyte solutions described above by comparing the peak heights with the level of noise present on the baseline Values of 300, 20, 8, 35, and 60 were obtained for CAPS/Arg, CHES/Arg, TAPS/Arg, MES/ Arg, and MOPS/Arg, respectively Best results were thus achieved for the CAPS/Arg solution even though the peak for the analyte in this solution was not quite the tallest Electrolyte solutions based on CAPS and Arg were therefore used for a more detailed investigation of As(III) determination Next, the molar ratios of CAPS and Arg were varied to examine the effect of changes of the pH value of the electrolyte solution on the determination It is shown by the results given in Fig that with an increase in the pH value of the electrolyte solution the peak heights increase (presumably due a higher degree of deprotonation of the arsenous acid) However, it was found that the baseline noise was also increasing with the pH value S/N ratios of 300, 210, 190, 195, 185, 100, and 75 were thus determined for the solutions with pH values of 9.0, 9.2, 9.4, 9.6, 9.7, 9.9, and 10.0, respectively, so that the best S/N was in fact obtained for the lowest pH value The short-term baseline noise (as opposed to longer term instabilities due to Joule heating), was found to show a correlation with the conductivity of the electrolyte solution, which increased with the pH value from 55 mS/cm (pH 9.0) to 135 mS/cm (pH 10.0) Furthermore, the separation from the carbonate peak is more pronounced for the electrolyte solutions with the lower pH values, a feature which is important for the large volume injection method discussed below It was also found that the baseline stability in the region of the carbonate peak deteriorated with time and a stepwise profile of the © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Electrophoresis 2007, 28, 3500–3506 Figure Electropherograms for a standard solution of 100 mM As(III) in different electrolyte solutions at pH 9.0 (30 mM CTAB was added to all buffers for EOF modification) Sample injection: hydrodynamic, 20 cm for 10 s Detection parameters: 300 Vp–p, 200 kHz Separation potential: –20 kV Capillary: fused silica, 50 mm id, Lt = 50 cm (Leff = 43 cm) (1) 50 mM CAPS/2 mM Arg; (2) 50 mM CHES/30 mM Arg; (3) 15 mM TAPS/20 mM Arg; (4) 12 mM MES/20 mM Arg; (5) 12 mM MOPS/20 mM Arg Figure Electropherograms for a standard solution of 100 mM As(III) in electrolyte solutions composed of CAPS/Arg at different pH values pH 9.0: 50 mM CAPS/2 mM Arg; pH 9.2: 20 mM CAPS/ mM Arg; pH 9.4: 20 mM CAPS/5 mM Arg; pH 9.6: 20 mM CAPS/ mM Arg; pH 9.7: 20 mM CAPS/10 mM Arg; pH 9.9: 20 mM CAPS/20 mM Arg; pH 10.0: 10 mM CAPS/20 mM Arg Separation and detection parameters as for Fig www.electrophoresis-journal.com Electrophoresis 2007, 28, 3500–3506 baseline was observed after several electrophoretic runs This effect was more pronounced for the higher pH values of the electrolyte solution, and it is assumed that this is related to the fact that the higher pH values are closer to the pKa2 value of carbonate (10.3), so that the fraction of doubly charged carbonate present is larger The reason for the pronounced shift in the elution times for carbonate and As(III) between the solutions of pH 9.9 and 10.0 is not clear, but was found to be reproducible An electrolyte solution with a pH value of 9.0 (corresponding to a ratio of 50/2 mM (CAPS/Arg)) was thus chosen for the determination of As(III) as overall the best performance was achieved with this electrolyte solution In order to obtain best baseline stability, the electrolyte solution in the two containers at the capillary ends was replaced before each run and the capillary itself rinsed with fresh electrolyte solution 3.1.2 Large-volume sample injection CE and CEC 3503 contain iron, the arsenic species can be adsorbed by and/or coprecipitated with iron(III) hydroxide As(III) was, therefore, not detected in any of the samples which had been brought to Switzerland from Vietnam but it was demonstrated that after spiking with As(III) solution, the determination of low concentrations of As(III) was possible in the matrix of these samples Electropherograms for one of the samples, before and after spiking with 1.5 mM As(III), are shown in Fig The analysis was carried out using the optimized electrophoretic conditions (50 mM CAPS/2 mM Arg, and 30 mM CTAB, pH 9.0) The pronounced elongations from the baseline observed for both samples (see Fig 3) are due to the major anions Even though the sample matrices are quite different, as evidenced by the electropherograms, the determination of arsenic at much lower concentrations is still possible in both backgrounds In Fig 3B the relevant sections of the electropherograms are shown at an enlarged scale for illustration of Sample injection was carried out in the hydrodynamic mode by elevating one end of the separation capillary to a given height for a specific time The injection height (20 cm) was kept constant in all experiments and the injection time was varied between 10 and 120 s Longer injection times resulted in higher peaks for As(III), note however, that, as would be expected, reduced peak resolution was observed for larger injected volumes, especially when real samples with high concentrations of other anions were injected An injection time of 60 s was chosen for CE measurements as this generally provided sufficient resolution and enabled also high sensitivity 3.1.3 Separations A calibration for As(III) was carried out in the range of 1– 100 mM and a correlation coefficient (r2) of 0.9975 was achieved (based on five points) The LOD value was determined as the As(III) concentration giving a peak height corresponding to three times the baseline noise and was found to be 0.29 mM The reproducibility of peak areas, given as RSD values for three consecutive injections of a standard solution, ranged between 0.1 and 6.8% (n = 3) in the concentration range of 1–100 mM of arsenic(III) Groundwater samples were taken from contaminated areas in Vietnam where a high occurrence of arsenic species was expected as evidenced by AAS measurements (samples VN1 and VN2 were taken in the Ha Nam provinces and VN3 to VN5 were taken in Ha Noi City (Thanh Tri district)) [44– 47] The water was collected in plastic bottles (which had been washed with a detergent solution, deionized water, 0.1 M nitric acid, deionized water, and finally three times with appropriate water sample) directly from the well or tap The supply tubing was flushed for 15 before the sampling took place As(III) is unstable at ambient conditions and is rapidly oxidized by oxygen in air to As(V) Moreover, when samples © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Figure (A) Determination of As(III) in a groundwater sample (VN4) from Vietnam The spiked concentration of As(III) is 1.5 mM (B) Enlarged sections of the electropherograms Electrolyte solution: CAPS 50 mM /Arg mM, 30 mM CTAB, pH 9.0 Sample injection: hydrodynamic, 20 cm for 60 s Detection parameters: 300 Vp–p, 200 kHz Separation potential: –20 kV Capillary: fusedsilica, 50 mm id, Lt = 75 cm (Leff = 68 cm) www.electrophoresis-journal.com 3504 H T A Nguyen et al Electrophoresis 2007, 28, 3500–3506 this point The unknown peak visible in the electropherograms next to that of the arsenic species, was also found to be present in tap water taken in the laboratory at the University of Basel, Rhine river water sampled in Basel and bottled water purchased in a local shop, but could not be identified Recovery values for the water samples taken in Basel, and for the well water samples from Vietnam, spiked with 1.5 mM As(III) are given in Table The results show that it is possible to determine As(III) reliably in real samples at this level 3.2 Analytical procedure for the determination of As(V) 3.2.1 Electrolyte system It was initially attempted to develop a method that would allow the determination of As(III) and As(V) concurrently in the same electrolyte solution The first two pKa’s of arsenic acid (H3AsO4), the species in which inorganic As(V) is present in aqueous solutions, are 2.2 and 7.1, therefore, at a pH value of as used for the determination of As(III) it is also possible to detect As(V) in anionic form Several of the electrolyte solutions that had been examined for the analysis for As(III) were thus also tested for As(V) Although high sensitivity could be achieved for As(V), it was found that a serious interference from other inorganic anions occurred Phosphate and carbonate, which are present in most samples, comigrated partially with As(V) and only marginal improvements of the separation resolution was achieved by varying the composition and/or concentration of the electrolyte solutions The closeness of the electrophoretic behavior of arsenate and phosphate (the size and pKa’s of the species are very similar) precluded the use of high pH electrolyte solutions for As(V) and, therefore, electrolyte solutions of low pH were investigated for the separation of these two species A preliminary investigation of the separation was performed using the modeling software PeakMaster 5.1 (http:// www.natur.cuni.cz/,gas) and acetic acid was chosen as electrolyte for the determination of As(V) Several reports have been published on the successful determination of inorganic anions in low pH electrolyte solution using CE-C4D (see, for example the recent reviews [37–39]) An EOF modifier was not used with this buffer solution, as the silanol groups of the separation capillary are protonated to a high degree at low pH values and the EOF is thus significantly reduced The dependence of the peak height for As(V), and the resolution between As(V) and phosphate, on the concentration of acetic acid is illustrated in Fig 4A According to these results, 45 mM acetic acid was chosen as optimal electrolyte concentration since good sensitivity and excellent resolution between As(V) and phosphate could be achieved An electropherogram for the two substances separated at these conditions is given in Fig 4B Moreover, no interference from carbonate was observed in the low pH electrolyte solutions The pKa values of carbonate are 6.4 and 10.3 and no analytical signal can be measured in acetic acid electrolyte solutions 3.2.2 Sample injection Injection parameters were adopted from the conditions reported for As(III) for which optimization of hydrodynamic injection was performed with respect to analyses of real samples and an injection time of 60 s for the capillary end elevated to a height of 20 cm was used 3.2.3 Separations Calibration for As(V) was carried out in the range of 0.5– 100 mM (based on six points) and a correlation coefficient (r2) of 0.9998 was achieved The LOD value was calculated based on the 36S/N criteria (by comparing peak height with baseline noise) and was determined to be 0.15 mM The RSD values for peak areas in the concentration range of 0.5– 100 mM were between 1.3 and 2.3% (n = 3) The ground water samples taken in Vietnam were also analyzed for As(V) As(V) was present in detectable concentrations in three of the five samples The electropherograms for the positive samples are given in Fig Differences in the matrix composition are also evident from the Table Determination of As(III) in spiked samples using an electrolyte solution of 50 mM CAPS/2 mM Arg and 30 mM CTAB as EOF modifier, a fused-silica capillary of 50 mm id, 75 cm total, and 68 cm effective lengths, and a separation voltage of –20 kV Sample Spiked concentration (mM) Concentration determined (mM) Recovery (%) Tap water River Rhine Bottled water VN1 VN2 VN3 VN4 VN5 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.57 0.06 1.32 0.09 1.79 0.06 1.45 0.07 1.64 0.12 1.58 0.02 1.65 0.03 1.68 0.08 105 88 120 97 109 106 110 112 © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.electrophoresis-journal.com CE and CEC Electrophoresis 2007, 28, 3500–3506 3505 Figure Determination of As(V) in ground water samples from different places in Vietnam Electrolyte solution: 45 mM acetic acid, pH 3.2 Separation and detection parameters as for Fig the method, the two samples from Vietnam without measurable As(V) as well as tap, Rhine river, and bottled water were spiked with standard and the recovery was calculated The results summarized in Table show that the quantification is reliable Figure (A) Optimization of the acetic acid concentration for peak height of As(V) and separation between As(V) and phosphate (n) Peak height for mM As(V) (m) Resolution R Separation and detection parameters as for Fig (B) Electropherogram illustrating the separation of phosphate and As(V) for a concentration of mM in 50 mM acetic acid Separation and detection parameters as for Fig electropherograms, but were of no concern for the current project The concentrations were determined as 0.16 0.01, 1.81 0.06, and 0.18 0.01 mM in the three samples VN2, VN3, and VN5, respectively Note that two of these values are close to the detection limit value given above The peak area integration employed for quantification of the samples (rather then peak heights) led to still acceptable precision values for these low concentration levels For verification of Concluding remarks The study presented is one of few to date to investigate the determination of trace levels of inorganic analytes in the presence of high levels of major ions using CE with contactless conductivity detection The determination of the inorganic anionic arsenic species of As(III) and As(V) was found possible in natural water samples with this simple and inexpensive method by using individually optimized methods These have potential in environmental applications and may be used for fast screening of groundwater contaminated with arsenic CE with contactless conductivity detection is a much simpler and less expensive approach then using ion-chromatography or atomic spectrometry methods, which also has Table Determination of As(V) in spiked samples using an electrolyte solution of 45 mM acetic acid, pH 3.2, a fused-silica capillary of 50 mm id, 75 cm total and 68 cm effective lengths, and a separation voltage of220 kV Sample Spiked concentration (mM) Concentration determined (mM) Recovery (%) Tap water River Rhine Bottled water VN1 VN4 1.00 1.00 1.00 1.00 1.00 0.99 0.03 0.92 0.02 1.08 0.02 1.08 0.08 1.10 0.05 100 93 108 108 110 © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.electrophoresis-journal.com 3506 H T A Nguyen et al the potential to be implemented in field portable instrumentation [48] Please note, however, that the detection limits achieved, expressed in conventional units 22 and 11 mg/L for As(III) and As(V), respectively, while adequate for the previous limits in potable water of 50 mg/L, are not quite adequate for all samples of interest, in particular considering the current guideline for drinking water which states a threshold level of 10 mg/L The authors would like to thank the Swiss Federal Commission for Scholarships for Foreign Students (ESKAS) to enable the postgraduate study for Huong Thi Anh Nguyen (Ref No.: 2005.0339/Vietnam/OP), the Swiss National Science Foundation (grant No 200020-105176/1 for partial financial support, as well as Michael Berg (Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Switzerland), and Pham Thi Kim Trang from the 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Schwarz, M A., Hauser, P C., Trends Anal Chem 2001, 20, 133–139 © 2007 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.electrophoresis-journal.com ... investigate the determination of trace levels of inorganic analytes in the presence of high levels of major ions using CE with contactless conductivity detection The determination of the inorganic. .. was demonstrated that after spiking with As(III) solution, the determination of low concentrations of As(III) was possible in the matrix of these samples Electropherograms for one of the samples,... achieved by varying the composition and/or concentration of the electrolyte solutions The closeness of the electrophoretic behavior of arsenate and phosphate (the size and pKa’s of the species